Optical communication systems have been and are of great commercial interest because of their high information carrying capacity. Such systems as presently contemplated use a light source which is optically coupled to a photodetector by a glass transmission line. The glass transmission line is commonly referred to as an optical fiber and is typically, at least at present, composed of a silica based composition. Such fibers presently exhibit extremely low loss and minimum chromatic dispersion within the wavelength range from 1.3 .mu.m to 1.6 .mu.m. Accordingly, although the original optical communication systems used wavelengths near 0.8 .mu.m, much interest has shifted to the longer wavelengths between 1.3 .mu.m and 1.6 .mu.m because of their potentially greater usefulness for high data rate, long haul communications systems.
The light source presently contemplated for use in most optical communication systems is a semiconductor laser diode. In attempts to obtain desirable device characteristics, such as low threshold currents and high modulation rates, many semiconductor laser diode structures have been proposed. One such structure is now termed a ridge waveguide laser by the skilled artisan and was first proposed and demonstrated in the AlGaAs materials system at a wavelength of approximately 0.8 .mu.m by Kawaguchi et al. See, for example, IEEE Journal of Quantum Electronics, OE-13, pp. 556-560, 1977. However, as interest in long wavelength systems increased as previously discussed, lasers capable of being modulated with very large bandwidths for applications in very high bit rate transmission systems have been sought at long wavelengths. Kaminow et al fabricated InGaAsP ridge waveguide lasers with impressive device characteristics. See, for example, Electronics Letters, 15, pp. 763-764, 1979. Lasers fabricated using the InGaAsP materials system can emit radiation in the desired long wavelength region. Kaminow obtained a remarkably flat frequency response to 4.5 GHz which was a direct result of not using reverse biased p-n junctions for current injection confinement as do many other laser structures.
However, present techniques for fabricating InGaAsP ridge waveguide lasers are unfortunately rather complex. In one representative embodiment, such lasers comprise an InGaAsP quaternary active layer and InGaAsP cladding layers on opposed major surfaces of the active layer. All epitaxial layers are grown lattice matched to an n-type InP substrate. It will be understood by the skilled artisan that the term InGaAsP as used herein refers to a materials system and that the active and cladding layers have different compositions. A p-type InP layer is grown on the cladding layer farthest from the n-type InP substrate. However, the p-type InP layer must be etched down to the InGaAsP anti-meltback or cladding layer to form the ridge waveguide. It is desirable that the waveguide have a width less than 5 .mu.m to ensure fundamental transverse mode operation. After etching, the top surface of the structure is covered with an insulator, such as silicon nitride, and a contact stripe window opened on the top of the narrow ridge. A final metallization is then made. As will be readily appreciated by those skilled in the art, these fabrication procedures require precise control of both the etching and the stripe alignment steps. Such precise control is often difficult to achieve.
Other device characteristics of lasers are also often of interest for communications systems. Although the spectral output of semiconductor diode lasers is relatively narrow as compared to that of, for example, light emitting diodes, the dispersion characteristics of the fiber are such that system capacity is frequently not maximized unless the laser emits radiation in a single longitudinal mode, i.e., the intensity of the unwanted modes is greatly suppressed with respect to the intensity of the desired mode. Such lasers are typically referred to by those skilled in the art as a single frequency laser although the spectral output has, of course, a finite width.
Several approaches have been taken in attempts to fabricate single frequency lasers. For example, there are coupled cavity lasers. Such lasers may have a single section laser with an external cavity or they may be a two-section diode laser with the two sections separated by, for example, a cleave. See, for example, W. T. Tsang, N. A. Olsson, R. A. Logan, Applied Physics Letters, 42, pp. 650-652, 1983 which describes a cleaved coupled cavity laser.
Another approach is frequency selective feedback in which the wavelength of the emitted radiation is selected by means of grating. In one version of a frequency selective feedback laser, the grating is fabricated close to the semiconductor active layer. Such lasers are commonly referred to by those skilled in the art as distributed feedback (DFB) lasers. Fabrication of DFB lasers is presently difficult because the composition and thickness of the active layer have to be precisely controlled with respect to the grating period as the grating should enhance radiation at the peak of the laser gain profile. Also, the grating quality must be preserved in all processing steps subsequent to its fabrication. This is often difficult because many fabrication techniques require growing semiconductor lasers directly over the grating. Of course, the problems previously discussed with respect to ridge waveguide lasers are also present in the fabrication of ridge waveguide DFB lasers.